Ajai Kumar Rai

Pleistocene oceanographie changes indicated by deep sea benthic foraminifera in the northern Indian Ocean

AbstractAn attempt has been made to understand the Pleistocene bottom water history in response to the paleoclimatic changes in the northern Indian Ocean employing quantitative analyses of deep sea benthic foraminifera at the DSDP sites 219 and 238. Among the 150 benthic foraminifera recorded a few species show dominance with changing percent frequencies during most of the sequence. The dominant benthic foraminiferal assemblages suggest that most of the Pleistocene bottom waters at site 219 and Early Pleistocene bottom waters at site 238 are of North Indian Deep Water (NIDW) origin. However, Late Pleistocene assemblage at site 238 appears to be closely associated with a water mass intermediate between North Indian Deep Water (NIDW) and Antarctic Bottom Water (AABW). Uvigerina proboscidea is the most dominant benthic foraminiferal species present during the Pleistocene at both the sites. A marked increase in the relative abundance ofU. proboscidea along with less diverse and equitable fauna during Early Pleistocene suggests a relative cooling, an intensified oceanic circulation and upwelling of nutrient rich bottom waters resulting in high surface productivity. At the same time, low sediment accumulation rate during Early Pleistocene reveals increased winnowing of the sediments possibly due to more corrosive and cold bottom waters. The Late Pleistocene in general, is marked by relatively warm and stable bottom waters as reflected by low abundance ofU. proboscidea and more diverse and equitable benthic fauna.The lower depth range for the occurrence ofBulimina aculeate in the Indian Ocean is around 2300 m, similar to that of many other areas.B. aculeata also shows marked increase in its abundance near the Pliocene/Pleistocene boundary while a sudden decrease in the relative abundance ofStilostomella lepidula occurs close to the Early/Late Pleistocene boundary.

Heavy mineral variation in the deep sea sediment of southeastern Arabian Sea during the past 32 kyr

The present study is based on heavy mineral assemblages (HM) of top 104-cm thick section of gravity core SK 221 (Lat. 8∘7.12′N; Long. 73∘16.38′E and water depth – 2188 m) located near the Chagos–Laccadive Ridge in the southeastern Arabian Sea to evaluate the provenance and paleoenvironmental changes during the last 32 kyr. The biogenic carbonate, acid insoluble residue, magnetic susceptibility, total organic carbon (TOC) and clay based humidity index, i.e., kaolinite/illite ratio are also utilized to correlate with the above paleoenvironmental changes. Ilmenite, garnet, staurolite, pyroxenes, andalusite and zircon are the dominant HM with moderate to low ZTR (zircon-tourmaline-rutile) index indicating instability of the sediments and rapid erosion in the source region. The characteristic HM suggest their mixed sources from the basic igneous, gneisses/granites, high grade metamorphic rocks and sandstones occurring mainly in the western and southwestern India. The temporal variations of HM, AIR (acid insoluble residue), MS (magnetic susceptibility), biogenic carbonate and Corg (TOC) during preglacial and early Holocene suggest intensive weathering, erosion, and transportation of terrigenous detritus from continental region by fluvial processes and summer monsoon led high biogenic productivity, respectively. The convective mixing of waters due to intense winter monsoon resulted in very high biogenic carbonate content during the early stages of glacial period. The HM and associated proxies indicated that the winter monsoons of Heinrich (H3, H2, and H1) and Younger Dryas (YD) events and summer monsoons of Bølling/Allerød (BA) event were not strong enough to bring drastic changes in the above parameters.

Deep sea benthic foraminiferal response to the Pliocene palaeoenvironments of the Northen Indian Ocean

The benthic foraminiferal contents of two DSDP sites 219 (Arabian Sea) and 237 (Mascarene Plateau)have been analysed in order to understand paleoceanographic changes in the northern Indian Ocean during Pliocene. Uvigerina proboscidea is the most dominant taxon with intervals of marked fluctuations in its abundance at both the sites reflecting relatively higher surface water productivity with changing magnitudes during Pliocene. Earliest Pliocene (c. 5.6 to 5.0 Ma) is the interval of distinct increase in the surface water productivity in response to more SW Indian Ocean monsoonal upwelling. The faunal assemblage during c. 5.0 to 4.0 Ma suggests relatively warm, oxygenated and more saline bottom waters and moderately low surface water productivity in the Arabian Sea region. This was also broadly the period of global warmth and higher eustatic sea level. The surface water productivity during c. 4.0 to 3.4 Ma at site 219 was moderately higher. However, site 237 is characterised by strong bottom currents and relatively lower surface productivity. Further, the latest Pliocene (c. 2.5 Ma onwards) is characterised by significantly higher surface water productivity induced by higher rates of SW Indian Ocean monsoonal upwelling which corresponds with the onset of major Northern Hemisphere glaciation. These paleoenvironmental changes are suggested to be mainly responsible for several last occurrences of benthic foraminiferal taxa near the Pliocene/Pleistocene transition.